1. Field of the Invention
The present invention generally relates to detection circuits which detect a minute change in capacitance responsive to a physical parameter for the purpose of measuring the physical parameter, and particularly relates to a detection circuit which detects a relative difference in the capacitances of a plurality of capacitors.
2. Description of the Related Art
Sensors that detect a minute change in the capacitance of a capacitor to measure a physical quantity causing such a capacitance change include a pressure sensor, an acceleration sensor, an angular rate sensor (gyroscope), etc. FIG. 1 is an illustrative drawing showing an example of the construction of a pressure sensor. A beam 13 provided inside a casing 10 for the pressure sensor partitions the interior of the casing 10 into a measured pressure room 14 in an upper half and a reference pressure room 15 in a lower half. Openings 10a and 10b are provided for the measured pressure room 14, thereby connecting an external gas pressure with an internal gas pressure inside the measurement pressure room 14. The reference pressure room 15 is sealed, and is filled with gas that generates a reference pressure. A detection electrode 11 provided on an inner wall of the casing 10 and a detection electrode 12 provided at a portion supported by the beam 13 constitute a capacitor comprised of a pair of opposing electrodes. A rise in the external pressure causes a pressure inside the measurement pressure room 14 to become greater than the pressure of the reference pressure room 15, thereby pressing down a portion of the beam 13 that is elastic, as shown in FIG. 2. This results in a capacitance change. Such capacitance change is electrically detected to measure the pressure.
A pressure sensor is generally implemented by use of a single capacitor as shown in FIG. 1. When calculating speed based on a difference between a static pressure (atmospheric pressure) and a dynamic pressure (pressure applied in the travel direction) as in an airplane or the like, two sensors each as shown in FIG. 1 are provided, and a relative difference between the capacitances of the two capacitors is detected. Accordingly, there is a need for a circuit that can accurately detect a relative difference in capacitance.
FIG. 3 is an illustrative drawing showing an example of the construction of an acceleration sensor. Detection electrodes 21a and 21b provided on an inner wall of a casing 20 for the acceleration sensor and detection electrodes 22a and 22b provided at a portion supported by a beam 23 constitute two capacitors comprised of two pairs of opposing electrodes. An weight 24 for use in detecting acceleration is attached to the beam 23. When acceleration is present, the weight 24 supported by the elastic beam 23 swings around the supporting point, tilting in response to acceleration as shown in FIG. 4. This causes a change in the relative capacitances of the two capacitors. This relative capacitance change is electrically detected to measure the acceleration. Accordingly, there is a need for a circuit that can accurately detect a relative difference in capacitance.
FIG. 5 is an illustrative drawing showing an example of the construction of an angular rate sensor (gyroscope). Electrodes 31a through 31c provided on an inner wall of a casing 30 for the angular rate sensor (gyroscope) and electrodes 32a through 32c provided at a portion supported by a beam 33 constitute three capacitors comprised of three pairs of opposing electrodes. The electrodes 31a and 31c and the electrode 32a and 32c are detection electrodes, and changes in the capacitances of the two capacitors formed by these electrodes are detected to measure angular rate. The central electrode 31b and the central electrode 32b are drive electrodes. They receive an alternating voltage to cause a simple harmonic motion of the weight 34 attached to the elastic beam 33 in a vertical direction as shown by an arrow A.
When angular rate is present with respect to the angular rate sensor (gyroscope), a Coriolis force is applied to the weight 34 that is exhibiting a simple harmonic motion. As a result, as shown in FIGS. 6A and 6B, the weight 34 tilts in response to the Coriolis force. Even when the angular rate is constant and maintains the same direction, a Coriolis force applied to the weight 34 that is moving downward (FIG. 6A) is in a different direction than a Coriolis force applied to the weight 34 that is moving upwards (FIG. 6B). Because of this, the side toward which the weight 34 tilts varies depending on whether the movement is upward or downward. Relative capacitance changes generated in this manner is electrically detected to measure the angular rate. Accordingly, there is a need for a circuit that can accurately detect a relative difference in capacitance.
FIG. 7 is a circuit diagram showing an example of the construction of a detection circuit that detects a relative capacitance difference in the case of an acceleration sensor. The detection circuit of FIG. 7 includes an oscillator 40, buffers 41 and 42, an XOR gate 43, and a low-pass filter 44.
FIG. 8 is a signal waveform diagram for explaining the operation of the detection circuit of FIG. 7. Signals A through E shown in FIG. 8 are identified with their respective positions in the circuit of FIG. 7. For the sake of convenience of explanation, signal waveforms B′ and C′ having cleaner shapes equivalent to the signals B and C are also illustrated. When the oscillator 40 of FIG. 7 generates the signal A comprised of repeated pulses as shown in FIG. 8, the signals B and C output from the respective buffers 41 and 42 are delayed according to the capacitances of the respective capacitors 45 and 46. The way such a delay occurs is shown as the time differences of rising edges and falling edges between the cleaner shaped signals B′ and C′ equivalent to the signals B and C.
The XOR gate 43 performs an exclusive logical sum between the signals B and C, thereby generating the pulse signal D having a pulse width responsive to the delay time difference. The low-pass filter 44 integrates the pulse signal D, thereby generating a DC voltage having a voltage responsive to the delay time difference. This delay time difference is attributable to a capacitance difference between the capacitors 45 and 46, which is in turn responsive to acceleration. Accordingly, the DC voltage output from the low-pass filter 44 has a voltage level responsive to the acceleration.    [Patent Document 1] Japanese Patent Application Publication No. 9-229784    [Patent Document 2] Japanese Patent Application Publication No. 10-227644
In sensors such as a pressure sensor, an acceleration sensor, an angular rate sensor (gyroscope), etc., a generated capacitance difference responsive to a physical quantity to be measured is minute. A conventional detection circuit such as that shown in FIG. 7 has a problem in that acceleration or the like cannot be accurately measured because a change in the output DC voltage becomes small when a capacitance change is small.
Accordingly, there is a need for a detection circuit which can accurately detect a minute capacitance difference.